When you do an Office 365 / EOL migration with Azure AD Sync in place, mailboxes may freeze up, where in the O365 console under “Mail Settings” for one or more mailboxes, it says “This user’s on-premises mailbox has not been migrated to Exchange Online. The Exchange Online mailbox will be available once migration is completed.” One may spend a whole lot of time, even with Microsoft on the line, not fixing this problem.
There may be more than one cause. But a very important cause and fix is outlined here:
The gist of it is, one of the user attributes synched up from the on-prem server is “msExchMailboxGuid”, and this is trouble, because if this is synched up, EOL thinks it is trying to make a duplicate of an existing mailbox, rather than a new one, and it will not make a duplicate, it is in fact waiting for this attribute to be deleted before proceeding.
So what we do, is we go to Synchronization Service, right-click on “Active Directory Domain Services”, make sure “Connector Designer” is selected, and then scroll down to msExchMailboxGuid , and uncheck it. Then click OK.
The next step is to kick off a sync. The easiest way is in Powershell on the server which has AD Sync installed:
Import-Module ADSync Start-ADSyncSyncCycle -PolicyType Initial
You can watch the progress in “Synchronization Service” if you like; after the two final exports are complete it is done. But we’re not done yet.
The next step, is to remove the licenses from all of the O365 accounts which are marked as not yet migrated.
Then run another sync.
Then add the licenses back.
And run another sync.
That did it this morning!